1. Field of the Invention
The present invention relates to a direct conversion receiving apparatus. More particularly, the present invention relates to a direct conversion receiving apparatus in which a reception high frequency signal is converted directly into a base band signal in frequency by a mixer using local oscillation signal frequency.
2. Description of the Related Art
Conventionally, a direct conversion receiving apparatus is known in which a reception high frequency signal is converted directly into a base band signal in frequency by a mixer using a local oscillation signal frequency (Japanese Laid Open Patent Applications (JP-A-Heisei 1-274518, JP-A-Heisei 3-16349,JP-A-Heisei 3-220823).
However, in this direct conversion receiving apparatus, a direct current (DC) off-set voltage is generated at the output of the mixer in addition to the required base band signal due to distortion of even numbered orders of the mixer. Also, when an operational amplifier is used as an amplifier of the base band signal, the DC off-set voltage is amplified by the amplifier. Therefore, in the conventional direct conversion receiving apparatus, reception quality (BER) is deteriorated and the dynamic range of an analog-to-digital converter is remarkably narrowed. Therefore, a method is proposed in which the above-mentioned DC off-set voltage is suppressed by use,of a feedback loop to extract and feed back a DC voltage component (Japanese Laid Open Patent Application (JP-A-Heisei 3-220823)).
In the above-mentioned conventional direct conversion receiving apparatus, circuits such as a duty detecting circuit and a digital-to-analog converter are necessary so that the apparatus has become large in scale on the whole. It is a severe problem especially in a portable phone in which the smallness and lightness of the apparatus are severely demanded. Also, it is a problem from the aspect of power consumption.
The present invention is accomplished to solve the above mentioned problems. Therefore, an object of the present invention is to provide a direct conversion receiving apparatus in which reception quality can be maintained in a good state without increase of power consumption.
Also, another object of the present invention is to provide a direct conversion receiving apparatus in which influence of a DC off-set voltage can be removed in a relatively small circuit structure.
In order to achieve an aspect of the present invention; a direct conversion receiving apparatus includes a receiving section for receiving a high frequency reception signal. A first frequency converting section includes a first capacitor, and frequency-converts the high frequency reception signal into a first base band signal using a first local oscillation frequency signal. Then, the first frequency converting section removes a DC component from the first base band signal using the first capacitor, and converts the first base band signal with the DC component removed into a first digital signal. A second frequency converting section includes a second capacitor, and frequency-converts the high frequency reception signal into a second base band signal using: a second local oscillation frequency signal which is different from the first local oscillation frequency signal by 90 degrees in phase. Then, the second frequency converting section removes a DC component from the second base band signal using the second capacitor, and converts the second base band signal with the DC component removed into a second digital signal. A demodulating section shapes and then demodulates the first and second digital signals to generate a demodulation signal.
The first frequency converting section includes a first high pass filter composed of the first capacitor, and the second frequency converting section includes a second high pass filter composed of the second capacitor. In this case, a cut-off frequency of each of the first and second high pass filters is lower than a predetermined value.
More specifically, it is preferable that the cut-off frequency of each of the first and second high pass filters is less than about 1% of a symbol rate of QPSK modulation when the high frequency reception signal is subjected to QPSK modulation. Or, it is preferable that the cut-off frequency of each of the first and second high pass filters is less than about 10% of a symbol rate of a frequency spectrum spread modulation, when the high frequency reception signal is subjected to a direct spectrum spread modulation.
Also, the first frequency converting section may include a first analog-to-digital converter, and the first high pass filter is formed from the first capacitor and an input resistance of the first analog-to-digital converter. Also, the second frequency converting section may include a second analog-to-digital converter, and the second high pass filter is formed from the second capacitor and an input resistance of the second analog-to-digital converter. In this case, a capacitance of the first capacitor is desirably set such that the cut-off frequency of the first high-pass filter is lower than the predetermined value, and a capacitance of the second capacitor is set such that the cut-off frequency of the second high-pass filter is lower than the predetermined value. More specifically, a capacitance of each of the first and second capacitors is desirably set based on the input resistance of the corresponding one of the first and second analog-to-digital converters such that the cut-off frequency of each of the first and second high pass filters is less than about 1% of a symbol rate of QPSK modulation when the high frequency reception signal is subjected to QPSK modulation. Or, the capacitance of each of the first and second capacitors is set based on the input resistance of the corresponding one of the first and second analog-to-digital converters such that the cut-off frequency of each of the first and second high pass filters is less than about 10% of a symbol rate of a frequency spectrum spread modulation, when the high frequency reception signal is subjected to a direct spectrum spread modulation.
In the above, each of the first and second frequency converting sections may orthogonally demodulate the high frequency reception signal to which QPSK modulation is performed, and each of the first and second frequency converting sections may orthogonally demodulate the high frequency reception signal to which a direct spectrum spread modulation is performed.
Also, the first frequency converting section may include a phase shifter for shifting the local oscillation frequency signal by 90xc2x0 in phase, a first mixer for converting the high frequency reception signal in frequency using the phase-shifted local oscillation frequency signal to output the first base band signal, the first capacitor used to: remove a DC component from the first base band signal, and a first analog-to-digital converter for converting the first base band signal with the DC component removed into the first digital signal. Similarly, the second frequency converting section may include a second mixer for converting the high frequency reception signal in frequency using the local oscillation frequency signal to output the second base band signal, the second capacitor used to remove a DC component from the second base band signal, and a second analog-to-digital converter for converting the first base band signal with the DC component removed into the first digital signal.
In order to achieve another aspect of the present invention, a method of generating a demodulation signal in a direct conversion receiving apparatus, includes the steps of:
receiving a high frequency reception signal;
frequency-converting the high frequency reception signal into a first base band signal using a first local oscillation frequency signal;
removing a DC component from the first base band signal;
converting the first base band signal with the DC component removed into a first digital signal;
frequency-converting the high frequency reception signal into a second base band signal using a second local oscillation frequency signal which is different from the first local oscillation frequency signal by 90 degrees in phase;
removing a DC component from the second base band signal;
converting the second base band signal with the DC component removed into a second digital signal; and
shaping and then demodulating the first and second digital signals to generate a demodulation signal.
The step of removing a DC component the first base band signal and the step of removing a DC component the second base band signal are performed by first and second high pass filters whose cut-off frequencies are lower than predetermined values. In this case, each of the cut-off frequencies is less than about 1% of a symbol rate of QPSK modulation when the high frequency reception signal is subjected to QPSK modulation. Instead, each of the cut-off frequencies may be less than about 10% of a symbol rate of a frequency spectrum spread modulation, when the high frequency reception signal is subjected to a direct spectrum spread modulation.
Also, the step of converting the first base band signal with the DC component removed into a first digital signal is performed by a first analog-to-digital converter, and the step of removing a DC component from the first base band signal is performed using a first capacitor and an input resistance of the first analog-to-digital converter,. and the step of converting the second base band signal with the DC component removed into a second digital signal is performed by a second analog-to-digital converter, and the step of removing a DC component from the second base band signal is performed using a second capacitor and an input resistance of the second analog-to-digital converter.